Saponification type (or solution type) rosin sizing agents have long been used in combination with aluminium sulfate as an internal paper sizing agent in acidic paper making. Such kinds of sizing agents are known to be less effective at a low addition ratio, and the effect is known to decrease at a high temperature or in a neutral pH region or in a closed water system. Emulsion type rosin sizing agents were developed to cancel such disadvantage of the saponification type rosin sizing agents. However, they are still less effective at a low addition ratio in sizing, and are not satisfactory.
To offset the disadvantages of the rosin sizing agents, a product derived by alkali-saponification of alkenylsuccinic acid has recently come to be used as a sizing agent because of its sufficient effect at a low addition ratio (U.S. Pat. No. 4,514,544). However, these sizing agents still have a disadvantage that the sizing efficiency is low in high temperature paper making or at paper making at around a neutral pH region.
Further, an emulsion type of alkenylsuccinic acid sizing agent is known which is derived by emulsifying alkenylsuccinic anhydride containing an emulsifier in a cationized starch solution or water at a low concentration of about 0.5 to 3% and is useful as a neutral paper sizing agent (U.S. Pat. No. 3,321,069).
The mechanism of action of alkenylsuccinic anhydride in neutral paper making is based on direct reaction of an anhydride group with a hydroxyl group of pulp and the fixation thereof onto pulp fibers to produce a sizing effect. Accordingly, in conventional neutral paper making, alkenylsuccinic anhydride has necessarily been added in an anhydride form to a pulp slurry. The alkenylsuccinic anhydride is highly reactive to water. Therefore, if the alkenylsuccinic anhydride is preliminarily emulsified and dispersed in water, it reacts with water in a short time to lose the anhydride group, thereby losing its function as a neutral sizing agent, and furthermore causing coagulation, precipitation, or separation of the emulsion owing to the change of the emulsion state in a process of conversion of alkenylsuccinic anhydride to alkenylsuccinic acid. Thus, an alkenylsuccinic anhydride type emulsion sizing agent for neutral paper making is storable only for several hours in an aqueous dispersion state. Therefore, it cannot be supplied commercially as an emulsion concentrate, and has to be emulsified just before paper making by an emulsifying machine. Moreover, at an acidic region employing aluminium sulfate as a fixing agent, the sizing efficiency develops slowly and is low immediately after paper making.
As described above, the insufficiency of the sizing effect of conventional alkenylsuccinic anhydride emulsions immediately after acidic paper making is considered to be due to the facts that the emulsion sizing agent is fixed in an unchanged acid anhydride form, undergoing slow reaction of the alkenylsuccinic anhydride with pulp in an acidic region, and long time is required in reaction of the alkenylsuccinic anhydride with water to form alkenylsuccinic acid to produce a sizing effect upon reaction with aluminum sulfate. Accordingly, if alkenylsuccinic acid preliminarily formed from alkenylsuccinic anhydride can be emulsified, rapid reaction thereof with aluminium sulfate and sufficient sizing effect are expected to be achieved.
Alkenylsuccinic acids, which are highly hydrophilic, cannot readily be emulsified. Therefore it is extremely difficult with conventional techniques to prepare the emulsion of the alkenylsuccinic acid which is storable stably for a long time in a high concentration. As described above, when alkenylsuccinic anhydride is emulsified with a conventional technique, the anhydride reacts with water in the emulsion to form alkenylsuccinic acid, giving an alkenylsuccinic acid emulsion. In the process of conversion of alkenylsuccinic anhydride to alkenylsuccinic acid, however, the emulsion state changes, giving no stable emulsion of alkenylsuccinic acid. In other words, even though an emulsion of alkenylsuccinic anhydride can be prepared temporarily in a high concentration, the alkenylsuccinic anhydride reacts with water in the emulsion to change into alkenylsuccinic acid, causing simultaneously coagulation, precipitation or separation without keeping a stable emulsion state, so that a stable emulsion cannot be obtained which contains alkenylsuccinic acid in a high concentration.
The inventors of the present invention made a comprehensive study to utilize the superior properties of alkenylsuccinic acid as a sizing agent in an emulsion type to solve the aforementioned problems. As a result, the present inventors have found that a suitably selected emulsifier and/or an anionic polymer type dispersant containing a proper monomer component and/or a hydrocarbon resin containing no acid group makes an emulsion extremely stable in storage and to produce excellent sizing efficiency in paper making even at a low addition ratio, at high temperature, and in about a neutral pH region, which could not be achieved by conventional saponified alkenylsuccinic acid type sizing agents, and completed the present invention.